1. Field of the Invention
The present invention relates to grate bars and grates for combustion furnaces and, in particular, to a fluid-cooled grate bar having a duct defined therewithin for guiding a coolant, a grate constructed with these grate bars, and a process for cooling a grate bar and a grate.
2. Description of the Prior Art
Conventional grates employed in combustion furnaces such as a garbage incineration plant are formed by rows of grate bars with each row being arranged one above another and extended transversely to the transporting or feeding direction of the combustible materials such, for example, as garbage. The grate bars are typically provided with openings such, for example, as slots, gaps, and holes so that air can be blown therethrough to aid combustion. The grate bars, besides transporting and mixing (known as stoking) combustible materials, also remove by-products of combustion. Additionally, the processes of drying, preheating, degassing, gasifying and carbon-burning are carried out successively on the grate bars.
The grate bars must provide desirable fire control and combustion characteristics, and be able to withstand harsh conditions in the use environment. These objectives, however, are difficult to meet. During use, wear is induced on the grate bars and grates by a myriad of environmental stresses from such, for example, as static and impact loading of massive combustible materials, caustic chemical attacks by hazardous substances, and thermal cycling from combustion to ambient conditions.
The wear on the grate bars depends substantially on their temperature during use. It is well known that the wear on the grate bars adversely affects the fire control and combustion characteristics of the grates and shortens the life expectancy of the grates. The operation of the furnace could be disrupted if the grams and grate bars experience structural failure.
Conventional measures taken to minimize the wear on the grates during combustion include supplying a portion of the combustion air --known as primary air --to cool the grates. According to this method, cooling is principally achieved by regulating the supply of combustion air. The remaining portion of the combustion air, or secondary air, is used to aid combustion of gases in the furnace chamber.
Fire control characteristics and/or quality of combustion of a garbage incineration plant can be affected by the composition of the garbage. In particular, domestic garbage destined for a garbage incineration plant often contains materials with elevated heat values such, for example, as recyclable glass, biodegradable substances or vegetable waste, and plastics. The elevated heat values and/or variations of the range of heat values of the garbage constituents increase the temperature of the grates --not only because of the increased intensity of the combustion, but also the decreased cooling caused by diminished air flow across the grates. The high heat values of the combustible garbage materials also tend to cause the combustion air to redistribute such that a larger quantity of combustion air is required for the combustion of gases in the furnace chamber.